Patent Literature 1, for example, discloses a control device for an internal combustion engine for calculating an intake air amount using a value obtained by correcting a value detected by an in-cylinder pressure sensor (hereinafter referred to as CPS) to its absolute pressure when an ignition timing for in-cylinder mixture is determined. The value detected by the CPS is a relative pressure, and therefore the value needs to be corrected to an absolute pressure in order to accurately obtain the intake air amount. An adiabatic process is conducted after an intake valve is closed and before the in-cylinder mixture is ignited in a compression stroke of the internal combustion engine. The control device calculates an in-cylinder pressure correction value by the following equation (1) using Poisson's equation established during the adiabatic process in order to correct the value to the absolute pressure.In-cylinder pressure correction value=(PbVbκ−PaVaκ)/(Vaκ−Vbκ)  (1)(where Pa and Pb are values detected by the CPS within the adiabatic compression stroke period after the intake valve is closed and before the in-cylinder mixture is ignited, Va and Vb are in-cylinder (combustion chamber) volumes when Pa and Pb are detected, and κ is a specific heat ratio.)